Process for modulating interferometric lithography patterns to record selected discrete patterns in photoresist

ABSTRACT

A double exposure process is disclosed whereby a first exposure produced by conventional photolithographic techniques generates a latent negative image in a photoresist etch mask layer ( 22 ), the image subsequently employed to modulate a second exposure generated by the multiple beam interferometric lithography technique. Periodic surface relief structures ( 80 ) patterned by the second exposure and formed after development of the exposed photoresist material, are restricted to regions ( 52 ) defined by the initial exposure, with the photoresist material ( 54 ) outside these regions remaining unmodulated, or devoid of the periodic structures ( 80 ), and suitable for use as a mask in a subsequent etching process.

This is a continuation of U.S. provisional application Ser. No.60/019,490, filed Jun. 10, 1996 abandoned.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a process for producing a plurality ofdiscrete arrays of sub-micron structures in a photoresist etch mask byinterferometric or holographic lithography techniques; each of thearrays is bounded by regions not subjected to the interferometriclithography.

2. Discussion of the Prior Art

Holographic or interferometric lithography is now a proven technologyfor creating structures having sizes smaller than a micron in acontinuous, two-dimensional, periodic array. For example, U.S. Pat. Nos.4,402,571, and 4,496,216, to Cowan, et al. and U.S. Pat. No. 5,142,385,to Anderson et al., the entire disclosures of which are incorporatedherein by reference, disclose methods and apparatus for producing aperiodic and continuous surface relief pattern in a surface by exposinga photosensitive material to a laser interference fringe pattern andthen developing the photosensitive material. Interferometric lithographyexploits the mutual coherence of multiple optical beams derived from asingle laser; the beams are overlapped in a selected region of space andinterfere to produce patterns of light and dark areas, or fringepatterns, repeating on a scale proportional to the laser wavelength. Thefringe patterns are recorded in photosensitive media such asphotoresist. Conventional contact or projection photomasks are notrequired and so interferometric lithography has become known as“maskless” lithography.

Interferometric lithography has been used in a laboratory environment inattempting to produce a flat panel display having a distributed cathode;the display is known as a Field Emission Display (FED). A FED is adistributed cathode, flat panel analog to the well known Cathode RayTube (CRT) and can include billions of microscopic cathode electron‘guns’ in an array distributed over the surface of a display substrate.Electrons emitted from the microscopic, cone shaped cathodes, under theinfluence of a large accelerating potential, strike a phosphor screendisposed opposite a common anode, and are thereby converted to photons(i.e., light). In making the cathode matrix in a FED, it has beendiscovered that the most critical fabrication step is patterning of anarray of high resolution features such as holes or cathode emitter tips.In the prior art, a photosensitive medium such as photoresist wasemployed to record an image of a hole array formed by a conventionalphotolithographic technique such as contact printing with shadow maskingtechniques, optical projection, or electron beam writing. The array ofholes in photoresist was then used as an etch mask in forming theemitter wells.

It would be desirable to use interferometric lithography in making anetch mask for fabricating FEDs, but the continuous nature ofinterferometric lithography fringe patterns is not suitable for use inan etch mask which must have cathode cone holes (or tips) only inpreselected pixel or sub-pixel regions. In other technologies, a similarproblem exists, for example, in making a Dynamic Random Access Memory(DRAM), Central Processing Unit (CPU) or a logic chip, high densitypatterns in an etch mask must be confined within or combined with otherpatterns for leadouts, contact vias or individual device area patterns.There is a need, therefore, for a method to selectively negate exposureto interferometric fringe patterns in areas outside selected regionssuch as the pixel region, but without a requirement for removing thephotoresist. There is also a need for a method or process for making anetch mask for producing FEDs which requires the fewest number of processsteps and which can be completed in the least amount of time, to satisfyeconomic requirements as dictated by the marketplace.

OBJECTS AND SUMMARY OF THE INVENTION

Accordingly, it is a primary object of the present invention to overcomethe above mentioned difficulties by providing a method for making anetch mask having a plurality of discontinuous and discrete arrayscontaining a high density of high resolution features created byinterferometric lithography.

Another object of the present invention is providing an efficient andeffective method for making an etch mask segmented in a selected numberof discontinuous subareas in which high resolution interferometriclithography can be used to provide sub-micron sized structures.

The aforesaid objects are achieved individually and in combination, andit is not intended that the present invention be construed as requiringtwo or more of the objects to be combined unless expressly required bythe claims attached hereto.

In accordance with the method of the present invention, patterns ofsub-micron structures in a photoresist etch mask are produced byinterferometric or holographic lithography techniques after image-wiseexposure using photolithographically generated pattern overlays. In thefirst step, negative pattern overlays are used to create a plurality ofsub-pixel regions of blocked or shaded photoresist bounded by a larger,rectangular region of exposed or illuminated photoresist. In the secondstep, the photoresist etch mask layer is chemically affected, eitherthermally or by flooding or immersion in a gaseous or liquidenvironment, such as saturation with ammonia vapor, thereby renderingthe formerly exposed rectangular region of photoresist insensitive tofurther light exposure and insoluble in subsequent etching steps. In thethird step, a sub-micron, high resolution light interference pattern ismodulated or apertured in the photoresist layer etch mask, in situ, bythe now insensitive, low resolution photoresist negative pattern,whereupon the light interference pattern causes periodic arrays ofsub-micron exposed spots only in sub-pixel regions of the lightsensitive photoresist. In the fourth step, the photoresist layer ischemically developed and the exposed spots are etched away, leaving aplurality of discrete (i.e., separate) periodic arrays of sub-micronholes in the etch mask.

The above and still further objects, features and advantages of thepresent invention will become apparent upon consideration of thefollowing detailed description of a specific embodiment thereof,particularly when taken in conjunction with the accompanying drawings,wherein like reference numerals in the various figures are utilized todesignate like components.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1a is an overhead view of a single pixel in a field emissiondisplay, illustrating method step one, an initial light exposuredelineating sub-pixel regions as defined by a cathode mesh adapted tocontain an array of half-micron holes to be patterned using aholographic technique in subsequent steps.

FIG. 1b is a cross-sectional view of the single pixel taken along lineA-A′ of FIG. 1a illustrating the initial light exposure to delineatesub-pixel regions.

FIG. 2a is an overhead view the single pixel, illustrating method steptwo, saturation with ammonia vapor of the photoresist bearing the latentimage formed by the exposure of the step of FIG. 1a.

FIG. 2b is a cross-sectional view of the single pixel illustrating thesaturation step taken along line A-A′ of FIG. 2a.

FIG. 3a is an overhead view of the single pixel, illustrating methodstep three, a holographic or interferometric exposure to form a latentimage of an array of half-micron areas of high luminous intensity.

FIG. 3b is a cross-sectional view of the single pixel illustrating theholographic exposure step taken along line A-A′ of FIG. 3a and showingthe placement of the half-micron areas of high luminous intensity.

FIG. 4a is an overhead view of the single pixel, illustrating theresulting photoresist etch mask layer after method step four, adevelopment step, leaving a plurality of periodic arrays of etch maskholes disposed only in the sub-pixel regions.

FIG. 4b is a cross-sectional view of the single pixel taken along lineA-A′ of FIG. 4a, illustrating the resulting photoresist etch mask layer,after developments.

DESCRIPTION OF THE PREFERRED EMBODIMENT

In accordance with standard industry practice, a layer of positivephotoresist material is coated onto a suitable substrate material suchas glass, silicon or sapphire.

Referring specifically to FIGS. 1a and 1 b of the accompanying drawings,an overhead view of a subsection of a field emission display substrate10 corresponds to the area required for a single pixel 12 including acathode mesh 14. FIG. 1a includes an imaginary planar layer 15 of lightand shaded regions illustrating incident light (or the lack thereof inan imaginary cross section. In a first method step using conventionalphotolithographic techniques such as shadow masking (i.e., contactprinting) or optical projection, an initial, negative pattern,image-wise exposure to light includes sixteen square blocked (i.e.,shaded or dark) areas 18 all within and bounded by an illuminated,substantially rectangular mesh-shaped area 20. The pattern shown in FIG.1a is a pixel area definition image, as is required during manufactureof a FED panel. Blocked or dark areas 18 delineate sub-pixel regions asdefined by cathode mesh 14. FIG. 1b is a cross-sectional view of thesingle pixel 12 taken along line A-A′ of FIG. 1a, and illustrates theinitial light exposure to delineate blocked sub-pixel regions 18 in a6000 Å thick photoresist layer 22.

As shown in FIG. 1b, display substrate 10 includes a substantiallyplanar glass base layer 30 having an upper surface 32 opposing a lowersurface 34. A 5000 Å thick Molybdenum and Silicon (Moly/Si) layer 36 isadhered to glass base layer upper surface 32 and includes sixteensquare, discrete Silicon (Si) sub pixel regions 38 all within andbounded by a Molybdenum (Moly) cathode mesh 14 electrically connected tothe cathode line 40 as shown in FIG. 1a. A 4000 Å thick Silicon Dioxide(SiO₂) layer 42 is disposed upon and adhered to an upper surface of theMoly/Si layer 36, opposite the glass base layer 30. A 1000 Å thickNiobium (Nb) gate layer 44 is disposed upon and adhered to an uppersurface of the SiO₂ layer 42, opposite the Moly/Si layer 36 and iselectrically connected to the gate line 45 as shown in FIG. 1a. Thephotoresist (PR) layer 22 is disposed upon and adhered to an uppersurface of the Nb layer 44, opposite the SiO2 layer 42, and has anuppermost surface 46, part of which is exposed to the light in region 48(i.e., corresponding to a portion of illuminated area 20 as seen inimaginary layer 15) during the first step. The exposure to the lightduring the first step chemically alters the composition of theilluminated and exposed area 20 of the photoresist layer 22, as is wellknown in the art.

As shown in FIGS. 2a and 2 b, the second method step is exposure orsaturation with ammonia vapor (illustrated schematically as layer 50) ofthe photoresist layer 22, now bearing the latent image formed by thelight exposure of the first method step illustrated in FIG. 1a. Thelatent image includes sixteen square, unaffected and previously blockedor shaded regions 52 all within and bounded by an affected, previouslyilluminated, substantially rectangular area 54 corresponding to theilluminated area 20 in FIG. 1a and 1 b. The unaffected regions 52correspond to the sub-pixel regions of blocked areas 18 in FIGS. 1a and1 b. In the second step, photoresist etch mask layer 22 is chemicallyaltered or affected, either by flooding or immersion in a gaseous orliquid environment (e.g., saturation with ammonia vapor 50 heated to atemperature at or above one hundred degrees Celsius for a period ofapproximately ten minutes) or thermally, as is known in the art, therebyrendering the formerly exposed rectangular region 54 of photoresistinsensitive to further light exposure and insoluble in subsequentdeveloping steps. The step of chemically affecting photoresist etch masklayer 22 may be carried out in an image reversal oven, a conventionalcomponent in most semiconductor fabrication facilities. Alternatively, achemical compound is added to photoresist layer 22 and heated, therebycausing previously exposed sections of the photoresist layer to crosslink.

Turning now to FIGS. 3a and 3 b, illustrating method step three, aholographic or interferometric exposure is utilized to form a latentimage of a periodic array of half-micron areas of high luminousintensity 62 separated by null areas of low luminous intensity 64. Inthe third step, a sub-micron, high resolution light interference pattern60 is modulated or apertured in the photoresist layer etch mask 22 , insitu, by the now insensitive, low resolution photoresist negativepattern 54, whereupon the light interference pattern 60 causessub-micron exposed spots 70 only in the light sensitive photoresist inthe sub-pixel regions 52. Light interference pattern 60 is a periodicand continuous laser interference fringe pattern created from the mutualcoherence of multiple optical beams derived from a single laser; thebeams are overlapped in a region of space just over the uppermostsurface 46 of the photoresist layer 22 and interfere to produce areas ofhigh luminous intensity 62 and areas of low luminous intensity 64 orfringe patterns, repeating on a scale proportional to the laserwavelength. The fringe patterns are recorded in a periodictwo-dimensional close-packed array of exposed spots 70 in only the stillphotosensitive sub-pixel regions 52 of photoresist layer 22.

In the fourth step as shown in FIGS. 4a and 4 b, photoresist etch masklayer 22 is chemically developed in accordance with standard industrypractice in a (preferably aqueous) liquid developer and the exposedspots 70 (FIG. 3b) are dissolved away, leaving sub-micron diameter rightcircular cylindrical holes 80 in and through the etch mask layer 22.Each etch mask hole 80 has a first open end 82 at the uppermost surface46 in fluid communication with a second open end 84 at the interfacebetween the photoresist layer 22 and the Nb layer 44. After thedevelopment step, a plurality of periodic arrays of etch mask holes 80are disposed in the separate sub-pixel regions 52. The etch mask holesare disposed only in the sub-pixel regions 52 and are not present in thesurrounding affected area 54; thus the sub-pixel regions are deemed tocontain discrete (i.e., spaced or separate) arrays of sub-micron etchmask holes 80.

For purposes of defining nomenclature, the method of the presentinvention uses the affected and insensitive area 54 to spatiallymodulate or to provide an aperture for use in the interferometriclithography steps to follow. The affected and insensitive area 54remains unmodulated or unperforated by etch mask holes and permits thephotoresist layer to be used as an etch mask in the subsequent etchingprocess used in finishing the FED.

The source of illumination used to initially expose the photoresistlayer 22 can be an optical image projector with an optical mask andlenses as is known in the art, or can include a shadow mask for contactprinting; alternatively, a scanning electron beam, scanning laser beamor proximity printing can be used. In each alternative, the process isan image reversal process using a negative image of a selected pattern;when using a scanning electron beam or scanning laser, the pattern maybe stored in software such that pattern software in a beam controllerdirects writing with the scanning (electron or laser) beam. In each ofthe above examples the photoresist layer is exposed using actinicradiation.

The method of the present invention may be characterized in generalterms as a method for producing an etch mask in a photoresist layer overa substrate (e.g., FED display substrate 10) for lithographic processingincluding the following steps:

1) controlling the locations at which a source of illumination shinesupon the photoresist layer (e.g., etch mask 22) by use of a firstpattern; where the first pattern defines a first selected region (e.g.,a first blocked sub-pixel area 18), a second selected region (e.g., asecond blocked sub-pixel area 18) and a third selected region (e.g., anilluminated area 20), where the first selected region and the secondselected region are bounded by the third selected region; and exposingthe photoresist layer to illumination from the source of illuminationsuch that illumination is not transmitted for the first pattern firstselected region and the second selected region and illumination istransmitted for the first pattern third selected region, whereby thephotoresist layer is not exposed in a first sub-area (e.g., regions 52)corresponding to the first pattern first selected region, and is notexposed in a second sub-area corresponding to the pattern secondselected region, and is exposed in a third sub-area (e.g., region 54)corresponding to the first pattern third selected region;

2) exposing the photoresist layer to a reactive environment (e.g., animage reversal oven containing a fluid such as ammonia vapor); reactiveenvironment exposure alters the photoresist layer in the third sub-areato an impervious state;

3) exposing the photoresist layer to a periodic pattern ofinterferometric illumination (e.g., pattern 60, by multiple laser beaminterferometry) and altering the photoresist layer in the first andsecond subareas with a periodic pattern of exposed spots (e.g., spots70), while the third sub-area is substantially insensitive to andunaffected by the interferometric illumination; and

4) developing the photoresist layer and removing the photoresistmaterial only in the spots exposed to the interferometric light andwithin the first and second subareas to make etch mask holes (e.g.,holes 80).

As an aside, it should be noted that in a completed FED, an anode (notshown) is customarily disposed in close proximity to the cathode mesh 14and includes a glass layer coated with a conductive material and aphosphor.

A number of variations are possible. For example, the thickness ofphotoresist layer 22 can be in the range of 1000 Å to 20,000 Å. Cathodemesh 39 can be any suitable conductor. Niobium Gate layer 44 can be anysuitable material which will preserve the gate function. Additionally,any arbitrarily selected region can be patterned as a first, larger area(e.g., pixel area 12) and subdivided into a plurality of subareas (e.g.,sub-pixel regions 18, 52).

Having described preferred embodiments of a new and improved method, itis believed that other modifications, variations and changes will besuggested to those skilled in the art in view of the teachings set forthherein. It is therefore to be understood that all such variations,modifications and changes are believed to fall within the scope of thepresent invention as defined by the appended claims.

What is claimed is:
 1. A method for confining interference lithographypatterning to discrete areas of a photoresist while maintaining asurrounding photoresist region that bounds the discrete areas,comprising the steps of: (a) exposing the surrounding photoresist regionwithout exposing the discrete areas of the photoresist; (b) treating thephotoresist to make the exposed surrounding photoresist regioninsensitive to further exposure and insoluble in a photoresistdeveloper; (c) selectively exposing the discrete areas of thephotoresist using interference lithography; and (d) developing thephotoresist to remove exposed photoresist within the discrete areas toform a repetitive pattern within the discrete areas without removing thesurrounding photoresist region, such that the surrounding photoresistregion remains as a protective etch mask that confines subsequent etchprocessing of an underlying layer to the discrete areas.
 2. The methodof claim 1, wherein step (a) includes exposing the surroundingphotoresist region using an exposure technique other than interferencelithography.
 3. The method of claim 1, wherein step (a) includes using ashadow mask to prevent exposure of the discrete areas while exposing thesurrounding photoresist region.
 4. The method of claim 1, wherein step(a) includes exposing the surrounding photoresist region using andelectron beam.
 5. The method of claim 1, wherein step (a) includesexposing the surrounding photoresist region with a laser beam.
 6. Themethod of claim 1, wherein step (b) includes applying a fluid reactiveagent to the photoresist.
 7. The method of claim 6, wherein the fluidreactive agent is ammonia vapor.
 8. The method of claim 1, wherein step(b) includes heating the photoresist to a temperature of at least 100°C.
 9. The method of claim 1, wherein the photoresist is part of alayered structure that includes discrete electrode regions formed in alower layer, wherein the discrete regions of the photoresist are alignedwith the electrode regions and the protective etch mask confines etchingto portions of layers underlying the discrete regions and overlying thediscrete electrode regions.
 10. The method of claim 9, furthercomprising forming a field emitter display by etching emitter tips inportions of the layered structure underlying the discrete areas of thephotoresist in correspondence with the discrete electrode regions.